Acinetobacter baumannii has become a serious nosocomial pathogen due to its persistence in the hospital environment and its multi-drug resistance patterns. Outbreaks of multidrug resistant A. baumannii (MDRAB) have been reported in many hospitals worldwide. What is surprising about these reports is many outbreads are polyclonal; that is, multiple phenotyically unique strains may appear in an outbreak. This is in contrast with outbreaks caused by other bacteria that are typically monoclonal caused by dissemination of a single strain of bacteria. From May to December 2007 a polyclonal outbreak of MDRAB occurred at the NIH Clinical Center. Analysis of these strains is underway to determine (1) why Acinetobacter outbreaks are typically polyclonal, and (2) if the outbreak strains have specific virulence factors that make them more virulent. [unreadable] [unreadable] A total of 45 A. baumannii clinical and surveillance isolates including 29 MDRAB were studied. Epidemiological relatedness of these strains was analyzed using rep-PCR, pulse field gel electrophoresis (PFGE) and multilocus sequence typing (MLST). [unreadable] [unreadable] A total of 17 different pulsotypes were identified, but only 4 were associated with MDRAB. Among these, one pulsotype presented a distinct temporal trend, displacing all other pulsotypes at the end of the outbreak. Rep-PCR was not able to differentiate most of the MDRAB. On the other hand MLST, showed a close relationship between three of MDRAB pulsotypes. This result is consistent with a common ancestor for the epidemic MDRAB strains in this outbreak. This increased genome plasticity within epidemic MDRAB might be associated with insertion elements a hypothesis that remains to be tested by whole genome sequencing.